The histology of a sauropod rib bone found in the Wessex formation, Hanover point, Isle of Wight.

In September 2015, I went to Compton Bay on the Isle of Wight to hunt for dinosaur bones. It was equinox tides all week, so an ideal time to get out on the furthest rocks of the Wessex formation, dating from the Barremian stage of the early Cretaceous (about 130mya) also famous for the bone debris beds, which are highly fossiliferous. Time passed and I hadn’t had a great amount of luck. So, deciding today was not my day, I decided to head home. As I turned, I glanced down to see a beautiful piece of rib bone with the most amazing internal structure I’ve ever seen (Fig. 1). But also nothing like I’ve ever seen before.

I took it show my tutor, David Martill, at the University of Portsmouth. He was quick to identify it as being from a sauropod, due to the large air cavities now filled in with a clear mineral banded by pyrite. He then followed the identification by: “how’d you fancy cutting it in half for a thin section?” I was dubious about the idea at first: I’d never looked at a bone and thought ‘you know what, that would be better cut in half’. But I went along with it and handed over my prize.

Fig. 1. The bone when found at Hanover Point, Isle of Wight, September 2015.

What is a thin section?

A thin section is an approximately 30µm thick slice of rock, or in this case, dinosaur bone attached to a glass slide with epoxy, a very strong, transparent glue. The thinness of the specimen makes it transparent, so can be viewed under a microscope. This allows you to identify individual minerals and microstructures of the bone.

Making a thin section: the process

First, the bone is cut in two using a rock cutting saw, which has diamonds embedded in the metal. The cutting is performed slowly and in stages to reduce the vibrations to the bone, reducing the risk of crumbling.

Once the bone has been cut, the chosen end is embedded with proxy resin (like superglue). This hardens the bone and ensures it will stay intact during the next stage of the process.

Fig. 2. Diagram of thin section slide construction.

The chosen piece is then attached to an arm, which holds it securely in place to the side of a small diamond tipped circular saw. This slowly cuts a slice of bone away approximately 1 to 2mm thick. The slice of bone has one side polished to remove the saw marks, and the same side is then stuck to a glass slide using epoxy.

However, the bone is still too thick to see through. So it is thinned down by grinding using successively finer sandpapers until it is roughly 30µm. A cover slip is then stuck on using epoxy, which protects the specimen from damage, as well as increasing the clarity under the microscope.

The thin section

A few days later, my bone came back, with the cut end polished and a beautiful thin section slide with it (Fig. 3). I was amazed to see how much of the bone contained these air cavities, which would continue to grow larger during the life of the sauropod down the length of the bone until the outer edge of the bone was millimetres thick. This would make the bone lighter in weight, and the large air cavities are also thought to have helped with the transport of oxygen around the body, ultimately allowing the sauropod dinosaurs to grow to such incredible sizes.

Fig. 3. The finished thin section slide.

I also noticed in the denser part of the bone a large patch that is a lot darker in colour compared to the rest of the bone. On further inspection, it was concluded that the colouring was probably due to the presence of original organic material preserved within the bone. It’s amazing to think that the material could survive for over 130myrs.

Bone histology

Down the microscope, it was evident early on just how good the preservation on this bone was (Fig. 4). Looking at the individual osteons (that is, roughly cylindrical structures found in the Haversian system), you can clearly see a round, white hole in its centre – this is the Haversian canal. These are a series of microscopic tubes that allow blood vessels and nerves to travel through the length of the bone.

In the body of the osteon, there are lacunae (that is, cavities or depressions), characterised here as dark specks. They are essentially small spaces containing a branched bone cell, called an osteocyte. These cells are capable of performing a variety of functions, such as molecular synthesis.

The lacunae are networked together by microscopic canals called canaliculi. Due to the high state of preservation in this specimen, these 1 to 2µm structures can be seen remarkably clearly.

Within the bone, there is evidence of lines parallel to the periosteum, known as LAGs (lines of arrested growth) (Fig. 5). These are created when there is abrupt metabolic disruption of bone formation, such as seasonal migration patterns. Therefore, theoretically, an individual’s age could be determined from them if a pattern was evident. However, this is unreliable due to reabsorption.

On this slide, reabsorption has been captured in time, most prominently by the osteons in the periosteum (that is, a dense layer of vascular connective tissue enveloping the bones). This process prevents the bone from becoming unnecessarily thick and, internally, it repairs micro breaks, so in turn strengthening the whole bone. This is especially important in ligament bones, as they undergo a lot of stress.

Perimineralisation

Looking at the air cavities, we can see that this bone was enriched several times, which would have started shortly after deposition. Water surrounding the bone, carrying dissolved minerals, infiltrates the microscopic pores and cavities. The minerals are deposited and crystallise through a process called perimineralisation.

From the several layers of different minerals present in the cavities between the trabeculae (that is, a small tissue elements in the form of a small beam, strut or rod, generally having a mechanical function), it is clear that perimineralisation occurred several times in this bone, (Fig. 6). First of all, pyrite formed the black bands around the edges of the trabecular. The presence of pyrite indicates a marine environment in which the bone was first deposited.

Pyrite is then followed by calcite, which forms the large colourless crystals growing inwards forming quite large, pointed intergrowth. This is followed by a growth of an altered amphibole, which fills the centre of the cavities. It was this repeated enrichment by minerals that preserved the bone to exceptional quality.

Overall, I am extremely impressed with how much a piece of bone can change and tell a greater story when it’s put under a microscope, and how much I have learnt from the whole process. And I have to say I am now very eager to find and cut up some more bones to find out what secrets they’re hiding.

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